Cigarette smoking is highly prevalent among persons with posttraumatic stress disorder (PTSD). Ambulatory and laboratory data collected during the last NCI R01CA8195 suggest that PTSD smokers' urges to smoke and smoking behavior may be particularly related to their symptomatology. Building upon this work and our newer pilot data, the current project will examine possible mechanisms of relapse in smokers with PTSD. Relapse continues to be a critical problem in smoking cessation treatment. Emerging evidence suggests that the majority of individuals attempting to quit smoking will relapse within the first or second week after quitting. Persons with PTSD may be at elevated risk for relapse due to a variety of factors including high levels of negative affect, attention problems, anxiety sensitivity, and poor distress tolerance or task persistence. Laboratory and ecological momentary assessment will be utilized to investigate antecedents of relapse and possible bio-behavioral predictors of relapse in smokers with PTSD. We propose to sample and compare smokers with and without PTSD, addressing the following three aims: 1) Investigate antecedents of smoking lapse and relapse, including ambulatory mood and PTSD symptoms; 2) Evaluate possible laboratory and psychological predictors of relapse including prepulse inhibition, anxiety sensitivity, measures of distress tolerance/task persistence and self-efficacy; and 3) Examine possible gender differences in ad lib smoking, antecedents of smoking lapse and relapse, and predictors of relapse. This study will potentially lead to benefits in multiple areas. First, this study should significantly advance knowledge about the antecedents and causes of smoking relapse in smokers with PTSD. Second, this information paired with a greater understanding of the mechanisms involved in ad lib smoking, may serve to inform development of specific smoking cessation strategies for this vulnerable population. Finally, by evaluating possible mechanisms that may be specific to or amplified in a psychiatric patient sample, we should gain a more complete understanding of the relationship between psychopathology and smoking behavior.